Developing a Method for Preparing Callovo-Oxfordian Claystone Samples at a Desired Damage Level in Triaxial Cell

For the deep geological disposal of radioactive waste in France, an Underground Research Laboratory has been constructed in the formation of Callovo-Oxfordian (COx) claystone at a depth of about 490 m. During the excavation, damage of host rock occurred around the openings. To investigate the hydro-mechanical behaviour of damaged COx claystone in the laboratory, it is necessary to propose a method for the preparation of damaged sample because field sampling appears difficult. In this study, such a method was proposed by performing triaxial test loading paths (constant mean stress and constant confining pressure). The anisotropic behaviour of COx claystone was revealed by isotropic loading. The strain–stress curve from triaxial tests exhibited elasto-plastic pattern. Both the shear modulus G and the Young’s modulus E decreased slightly with the increase of deviator stress, suggesting the damage behaviour. Interestingly, all results from this study and from the literatures showed that at 90% of the peak deviator stress qmax, the ratio ET,0.9/EI (where ET,0.9 represents the tangent modulus at 90% of qmax and EI represents the initial tangent modulus) is close to 0.22, suggesting that the tangent modulus ET is a good indicator of damage level for the COx claystone sample. Observation using X-ray computed tomography and Mercury intrusion porosimetry showed that the damaged sample is characterised by the creation of significant micro-cracks after the triaxial loading. This finding suggests that ET can be used to control the damage level in triaxial cell for preparing damaged COx claystone samples.